I used Lookout for years without issue, and it gets high marks by most users. As stated above, if you don't sideload apps from third parties, chances are slim you will encounter malware. However, there have been occurrences in the Play Store where malicious apps were available temporarily, until Google discovered and removed them.
Understand that malicious apps usually aren't destructive like computer viruses, i.e., deleting everything on your hard drive. The malware usually installs hidden software that tracks you, makes calls, or sends texts without your knowledge. Ironically, if you read the permissions of apps before installing them, some "legitimate" apps can do this too. The reason they aren't flagged as malware is because they're admitting they do it, and you are essentially giving them permission to do so.